As methods for forming a uniform thin film over a wide area with excellent reproducibility, vapor phase deposition methods, such as metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and sputtering, are well known. These methods are important as an industrial thin-film forming method. As a method for observing, in situ, an optical constant, a growth rate, etc. of a thin film formed by these vapor deposition methods, a method for monitoring change in light reflectivity with time is known. In this method, light beam is radiated to a measurement target formed with a thin film through an optical window provided on a wall surface of a thin-film forming apparatus to measure a reflectivity of light beam in a predetermined wavelength during a film formation process. When the surface of a substrate, on which a thin film is to be formed, is a mirror surface, for light radiated to the thin film, due to an interference effect between reflected light on the surface of the formed thin film and reflected light on the interface between the substrate and the thin film, an observed reflectivity periodically varies with respect to the thin-film thickness. The optical constant, film thickness, etc. of a formed thin film can be calculated from the cycle of reflectivity change to the film thickness, the minimum and maximum values of the reflectivity, and so on. Moreover, the growth rate can be calculated from a thin-film forming time.
A popular procedure of the above-described calculation will be explained hereinbelow. A thin-film reflectivity can be measured by a reflectometer. On the contrary, reflectivity change with time can be simulated with calculation of an appropriate function (hereinafter, referred to as a reflectivity model function, as required) using parameters, such as a growth rate, a refractive index, etc. of a formed thin film. By determining (fitting) a result of simulation using the above-listed parameters so as to have a minimum error in comparing the result of simulation with measured values of reflectivity change with time by the reflectometer, a fitting parameter in the reflectivity model function can be selected. However, a plurality of error minimum points may appear in the case of fitting the reflectivity model function to reflectivity measured values, and hence it is not easy to determine which minimum point is the correct solution.